The production of three-dimensional articles of complex shape by means of stereolithography has been known for a number of years. In this technique the desired shaped article is built up from a radiation-curable composition with the aid of a recurring, alternating sequence of two steps (a) and (b). In step (a), a layer of the radiation-curable composition, one boundary of which is the surface of the composition, is cured with the aid of appropriate imaging radiation, preferably imaging radiation from a computer-controlled scanning laser beam, within a surface region which corresponds to the desired cross-sectional area of the shaped article to be formed, and in step (b) the cured layer is covered with a new layer of the radiation-curable composition, and the sequence of steps (a) and (b) is repeated until a so-called green model of the desired shape is finished. This green model is, in general, not yet fully cured and may therefore be subjected to post-curing, though such post curing is not required.
Via an equivalent process, photopolymer can be jetted by ink jet or multiple ink jet processes in an imagewise fashion. While jetting the photopolymer or after the photopolymer is applied, actinic exposure can be provided to initiate polymerization. Multiple materials (for example non-reactive waxes, weakly reacting photopolymers, photopolymers of various physical properties, photopolymers with various colors or color formers, etc.) can be jetted or applied to provide supports or alternate cured properties.
The mechanical strength of the green model (modulus of elasticity, fracture strength), also referred to as green strength, constitutes an important property of the green model and is determined essentially by the nature of the stereolithographic resin composition employed in combination with the type of stereolithography apparatus used and degree of exposure provided during part fabrication. Other important properties of a stereolithographic resin composition include a high sensitivity for the radiation employed in the course of curing and a minimum amount of curl or shrinkage deformation, permitting high shape definition of the green model. In addition, for example, it should be relatively easy to coat a new layer of the stereolithographic resin composition during the process. And of course, not only the green model but also the final cured article should have optimum mechanical properties.
The developments in this area of technology move towards compositions having better mechanical properties in order to better simulate properties of commodity materials like polypropylene and engineering type polymers. Also there exists a requirement for faster cure and process speeds, so as to decrease the time to build a part. This has resulted in new stereolithography machines having solid state lasers that have a high energy output, very fast laser-scanning and fast recoating processes. The new machines supply UV light with a power around 800 mW and above, compared to 200-300 mW for the older conventional machines. Also the scanning time is reduced by 3 to 4 times. These high powers, high scanning speeds, and short recoating times result in higher temperatures, due to polymerization exotherm of the resins and parts during fabrication. Typical temperatures have risen to values between 50° C. and 90° C., which leads to part distortion and excessive color development.
In order to achieve the desired balance of properties, different types of resin systems have been proposed. For example, radical-curable resin systems have been proposed. These systems generally consist of one or more (meth)acrylate compounds (or other free-radically polymerizable organic compounds) along with a free-radical photoinitiator for radical generation.
Another type of resin composition suitable for this purpose is a dual cure type system that comprises (i) epoxy resins or other types of cationically polymerizable compounds; (ii) cationic polymerization initiator; (iii) acrylate resins or other types of free-radically polymerizable compounds; and (iv) a free radical polymerization initiator.
Separately, oxetane compounds have been suggested as components for stereolithographic resins and other radiation-curable resins. They have been suggested as either a cationically polymerizing organic substance or as a reactive modifier component for such resins.
Several references teach the use of oxetane compounds in radiation curable resin compositions, including the following:
U.S. Pat. Nos. 5,434,196 and 5,525,645 (Ohkawa et al.) are directed to resin composition for optical molding which comprises (A) an actinic radiation-curable and cationically polymerizable organic substance and (B) an actinic radiation-sensitive initiator for cationic polymerization.
U.S. Pat. No. 5,674,922 (Igarashi et al.) teaches active energy beam-curable compositions which comprise (A) at least one oxetane compound (B) at least one epoxide compound and (C) at least one cationic initiator.
U.S. Pat. No. 5,981,616 (Yamamura et al.) teaches photo curable compositions that contain (A) an oxetane compound (B) one or more selected epoxy compounds and (C) a cationic photo-initiator.
U.S. Pat. No. 6,127,085 (Yamamura et al.) teaches a photo-curable composition comprising (A) a specific epoxy compound having a cyclohexane oxide; (B) a cationic photo-initiator; (C) a specific ethylenically unsaturated monomer; (D) a radical photo-initiator; and (E) a polyol.
U.S. Pat. No. 6,136,497 (Melisaris et al.) teaches a method for producing three-dimensional shaped articles with a radiation-curable composition containing (A) 20-90% by weight of cationically polymerizing compounds; (B) 0.05-12% by weight of cationic initiator; and (C) 0.5-60% by weight of at least selected cationic reactive modifiers.
U.S. Pat. No. 6,368,769 (Ohkawa et al.) teaches a stereolithographic resin composition that may include mixtures of the following: (A) cationically polymerizable organic substance that could be a mixture of an epoxy compound and an oxetane compound (3-ethyl-3-hydroxy methyloxetane is mentioned as an oxetane compound); (B) selected cationic photo-initiator; (C) radically polymerizable organic substance such as a polyacrylate; (D) radical photo-initiators; and (E) optional organic compounds having two or more hydroxyl groups per molecule (e.g., polyethers).
U.S. Pat. No. 6,379,866 (Lawton et al.) teaches a photosensitive composition comprising (A) 30-70% by weight of a cycloaliphatic diepoxide; (B) 5-35% by weight of an acrylic material selected from aromatic acrylic material or combinations thereof; (C) 10-39% by weight of an aliphatic polycarbonate diol or polytetrahydrofuran polyether polyol; (D) at least one cationic photoinitiator; and (E) at least one free-radical photoinitiator.
U.S. Pat. No. 6,413,696 (Pang et al.) teaches liquid, radiation-curable compositions that contain (A) 55-90% by weight of at least one solid or liquid actinic radiation-curable and cationically polymerizable organic substance (these may include oxetane compounds, see column 6, lines 42 to 54); (B) 0.05 to 10% by weight of an actinic radiation-sensitive initiator for cationic polymerization; (C) 5% to 25% by weight of an actinic radiation-curable and radical-polymerizable organic substance; (D) 0.02 to 10% by weight of an actinic radiation-sensitive initiator for radical polymerization; and (E) 0.5 to about 40 percent by weight of at least one solid or liquid cationically reactive modifier-flexibilizer, wherein the reactive modifier-flexibilizer is a reactive epoxy modifier, reactive vinylether modifier, reactive oxetane modifier, or mixtures thereof, and wherein the reactive modifier-flexibilizer contains at least one chain extension segment with a molecular weight of at least about 100 and not more than 2,000, wherein component (A) comprises at least one glycidylether of a polyhydric aliphatic, alicyclic or aromatic alcohol having at least three epoxy groups with epoxy equivalent weight between 90 and 800 g/equivalent and at least one solid or liquid alicyclic epoxide with epoxy equivalent weight between 80 and 330 having at least two epoxy groups with a monomer purity of at least about 80% by weight, or mixtures thereof.
European Patent No. 0848294 B (DSM N.V.; Japan Synthetic Rubber Col, LTD. and Japan Fiber Coatings, Ltd.) teaches a process for photo-fabricating a three-dimensional object by selectively curing a photo-curable composition comprising an (A) oxetane compound, (B) an epoxy compound and (C) a cationic photo-initiator wherein the oxetane compound (A) is either a compound comprising two or more oxetane rings or a specifically defined oxetane compound.
Japanese Published Patent Application (Kokai) No. 1-0158385 (Asahi. Denka Kogyo KK) teaches a resin composition for optically three-dimensional molding containing a cationic polymerizable organic material containing an oxetane ring in its molecule.
US 2004/0137368 (Steinmann) teaches a liquid radiation curable composition that comprises cationically polymerizable substances, radically polymerizable substances, a hydroxyl functional component and at least one hydroxyl-functional oxetane compound.
There exists a need for a liquid stereolithographic composition that possesses very high reactivity, high photo speed with high green strength after cure, low viscosity, low humid sensitivity, and produces cured articles with high temperature resistance along with other mechanical and chemical properties desired in stereolithographic resins.